Radioactive burglar alarm system



Nov. 14, 1961 Filed Dec. 2, 1958 M. H. A. LINDSAY RADIOACTIVE BURGLAR ALARM SYSTEM FIG. I

FIG. 2

2 Sheets-Sheet 1 Nov. 14, 1961 M. H. A. LINDSAY 3,009,138

RADIOACTIVE BURGLAR ALARMlSYSTEM Filed Dec. 2, 1958 2 Sheets-Sheet 2 mmiom VVVVV AAAA A vvvvvv AAllAl lAAAl VVIVII United States Patent )fifice 3,009,138 Patented Nov. 14, 1961 3,009,138 RADIOACTIVE BURGLAR ALARM SYSTEM Maxwell H. A. Lindsay, New Providence, N.J., assignor to American District Telegraph Company, Jersey City, NJ., a corporation of New Jersey Filed Dec. 2, 1958, Ser. No. 777,723 5 Claims. (Cl. 340275) The present invention relates to burglar alarm systems and more particularly to burglar alarm systems of the type used for protecting doors, windows and the like against unauthorized intrusion.

Over the years various items of equipment have been used in the protection of doors, windows and other openings, for producing an alarm when the doors, windows or other openings are moved. Mechanically actuated contacts and magnetically actuated contacts, in particular, have evolved to a point where they may be used with excellent results in most protection situations. There is, however, one class of service in which satisfactory protection has been very difficult to achieve and this has been in the protection of large metal roll-curtain doors of the type commonly used in industrial and warehouse structures. It is to this class of service to which the invention is particularly directed and in which lies its greatest utility.

In protecting ordinary doors little difiiculty is encountered in applying conventional mechanically or magnetically actuated alarm contacts in a position in which they are both effective and trouble-free. Indeed, such contacts can be found on the doors of many thousands of mercantile and other establishments. Large sliding or sectional type wooden overhead doors can also be protected conveniently by conventional contacts, because contacts can be fastened in out-of-the-way places on such doors where they will be free from damage or contamination.

Roll-curtain doors of the type commonly used in industrial and warehouse buildings present a number of peculiar difiiculties which have made it difiicult to achieve really satisfactory protection despite numerous attempts to use a variety of devices. One diificulty is that generally the entire door rolls up like a window shade and only the bottom angle member remains outside of the roll housing, so that the bottom angle member has been the only portion of the door on which any device could be mounted for use in actuating associated protective contacts. But any device mounted on the door or frame at or near floor level is subject to a variety of disadvantages including mechanical damage from vehicles, workmen and the like, and damage due to dirt, corrosion and other undesirable operating conditions. Water from rain storms, floods, hosing and other sources has also presented difficulties in connection with contacts mounted near the floor. netic contacts suffer from the additional disadvantage caused by the deleterious effect upon the magnetic field caused by the iron of the door itself or of the iron channel in which such doors are generally mounted.

Another important problem encountered with rollcuitain doors is that such doors, especially when they have been in service for a relatively long time, tend to display some variation in position when closed so that contacts used for their protection must be able to accommodate such variations.

In all protection situations resistance to malicious defeat is, of course, important. Especially in connection with warehouses and the like it is desirable that the defeat resistance extend also to situations in which an inside accomplice attempts to aid in defeating the protection. With the protection devices heretofore used on large roll-curtain doors it has been diflicult to afford adequate resistance to well-trained intruders, and this has been particularly so when the intruders have been aided by a person with knowledge of and access to the protective devices.

The principal object of the present invention has been to provide a novel and improved burglar alarm system for use in connection with roll-curtain doors and which overcomes the foregoing problems.

More particularly it has been an object of the invention to provide a burglar alarm system using elements which will not interfere with door operation and which may be located well above the floor level so as not to be accessible to mechanical damage and so as to be relatively free of dust, dirt, corrosion, etc.

Another object of the invention has been to provide a novel and improved burglar alarm system for use in connection with doors and the like and which is operable through steel framework without modifying the construction of the framework.

A further object of the invention has been the provision of a novel and improved alarm system for signalling the movement of a movable element relative to a fixed element.

Another object of the invention has been the provision of a novel and improved burglar alarm contact in which the actuating element is a radioactive source.

Still another object of the invention has been the provision of a radioactive burglar alarm contact which is resistant to defeat even from substitute radioactive sources.

A further object of the invention has been the provision of a radioactive burglar alarm contact which can easily be adjusted to accommodate different ranges of variations in closed position of doors and especially large metal roll-curtain doors.

Other and further objects, features and advantages of the invention will appear more fully from the following description.

The system of the invention comprises a radioactive source mounted on the movable part of a building closure, a radiation detector mounted on a fixed part of the building so as to be in alignment with the source when the closure is in its closed position. The detector may be shielded so that radiation from the source can enter the detector only in a relatively narrow beam. The output of the radiation detector, which is preferably a Geiger tube, is in the form of electrical impulses. The number of these impulses which occur in any given interval of time will -be dependent upon the alignment and spacing of the detector and source, and also upon the presence of other radioactive sources. The detector and output impulses are integrated over some convenient short-time interval,

so as to provide an average signal level. Increases or decreases in the average signal level in excess of some pre determined value are used to transmit an alarm. Most satisfactory protection will generally be achieved when the alarm is transmitted to a central station where trained personnel are provided to respond to alarms.

An important advantage of the invention is that the radiation source and detector can be mounted without the expensive alterations to doors and frames which have often been necessary with other types of contacts. Thus since the system of the invention can operate through the framework even when the latter is iron, the cutting, sawing and other difficult work often required with other types of contacts is not needed in installing the system of the invention.

While the invention is particularly applicable to rollcurtain doors and the like, it can be used to advantage with other types of building closures and even in other protection situations. For example, the invention may be used to trap a safe or cabinet to provide a signal when the safe or cabinet is moved. For this purpose the radioactive source may be placed within the safe or cabinet and the detector may be mounted outside the safe or cabinet in alignment with the source.

The invention will now be described in greater detail with reference to the appended drawings in which:

FIG. 1 is an elevational view of a typical roll-curtain door showing the radioactive contacts of the invention installed thereon;

FIG. 2 is a sectional view taken along the line 2-2 of FIG. 1;

FIG. 3 is a perspective view illustrating the radiation detector housing; and

FIG. 4 is a schematic diagram illustrating the circuit of the invention.

Referring now to the drawings and more particularly to FIGS. 1 and 2, there is shown a typical industrial steel rollcurtain door mounted on a shaft 11 located within a steel hood 12 and arranged to roll up and down in the manner of a window shade under thecontrol of a hand chain diagrammatically indicated at 13. The shaft 11 is mounted in suitable brackets affixed to the building walls B. The sides of the door 10 extend into channel members 14 and 15 which, in the arrangement illustrated, are spaced outwardly of the building will by brackets '16 and 17. The channels 14 and 15 guide the door as it moves up and down, and prevent appreciable motion of the door inward or outward of the door opening 18. Instead of a face of wall mounting, as shown, many such doors are mounted between the jarnbs, in which case the channels 14 and 15 are ailixed to the build: ing wallsinside the door opening. i

A radioactive source 19 is provided on the door 10 adjacent one edge thereof and at a height above the floor (with the door closed) 'such that the associated detector 20 will be free from likelihood of accidental damage caused by vehicles, workmen, etc., using the door for egress and ingress. The detector 20 is mounted on the rear of the channel -14 so as to be in horizontal and vertical alignment with the source 19 when the door'10 is completely closed; With a between-jamb door mounting,

material might be 10 micrograms of radium. A radia-' tion strength of the order of 10 microcuries has been found satisfactory for safe handling and for adequate penetration through the'usual steel door side channel. The source-containing metal cylinder can be glued to the door surface and is small enough not to interfere with door operation. Preferably however the cylinder is afiixed by glue inside a hole drilled in the door 10 adjacent one side thereof.

The detector 20 comprises a lead housing 21 and a radiation detector 22 (FIG. 4) located within the housing 21. The housing 21 may be cylindrical or of any other convenient shape. A hole 23 is provided in the housing 21 in direct alignment with the source 19 when the door 10 is in closed position. The shield 21 limits the angle at which radiation can be directed onto the detector 22. In this way, with the door 10 closed, radiation from the source 19, after passing through the channel 14 and bracket 16, will enter the housing 21 in a narrow beam through the hole 23. As the door 10 is opened the alignment between the source 19 and the hole 23 will be lost and the radiation entering the hole 23 will be reduced. This reduction will be greater than the reduction which would result from merely increasing the distancebetween the source 19 and the detector 22.

The radiation detector 22 is preferably a Geiger tube and might be, for example, an Amperex 75MB3 Halagonquenched Geiger tube.

The cathode of the Geiger tube 22 is connected to ground as shown at 24 (FIG. 4). The anode of the Geiger tube 22 is connected to source of high D.C. voltage, e.g., 710 volts, through a resistor 25. The Geiger tube operating voltage is provided at a high-voltage terminal of a power supply 26. The power supply 26 and the other portions of the FIG. 4 circuit may be located at any convenient place near the door opening 18. Normally they will be mounted in a tamper-protected steel cabinet mounted on the building wall near the door opening 18. Two leads only need to be run from the housing 20 to the remaining portions of the circuit.

The output of the Geiger tube 22 will be a series of voltage impulses the number of which per unit time will be proportional to the radiation entering the Geiger tube. With the door 10 closed, the number of impulses per unit time will be equal to some selected value. As the door starts to open, the number of voltage impulses will decrease sharply until, with the door 10 open a few inches, the voltage impulses will decrease to a number corresponding to the normal background radiation. If an intruder should bring up an auxiliary source to simulate the source 19 as the door 10 opened, the number of voltage impulses per unit time would go up. By reason of the collimation afforded by the hole 23 in the lead shield 21, it will be virtually impossible to synchronize the motion of a. substitute source and the door 10 to maintain a constant level of radiation strength on the Geiger tube 21.

The voltage impulses from the Geiger tube 20 are applied to the control grid of a tube 27 through a capacitor 28. The tube 27 might conveniently be one half of a 5691 double triode. The grid of tube 27 is coupled to the B+ (250 volt) power supply terminal through a resistor 29 and to ground. potential through a resistor 30.

The. cathode of tube 27 is coupled to ground through a. resistor 31, while theanode is coupled to B+ through a resistor 32. The amplified output of tube 27 is supplied through a capacitor 33 to the control grid of a tube 34, which. might conveniently be the other half of the 5691 tube. The cathode of tube 34 is connected directly to the cathode. of tube 27, both tubes being biased by the current flow through resistor 31. The grid of tube 34 is coupled to ground. through a variable resistor 35 which serves as a system level control. is coupled to B-lthrough a series connection of a resistor 36 andv a coil 37.

Thecoil 37- is the main or meter coil of a relay meter 38. Current flow through coil 37 controls an armature 39, the arrangement being such that with a predetermined average current flow through the coil 37 the armature 39 will bemaintained in a position midway between contacts 40 and 41. With. an increase in average current, thearmature 39 will approach the contact 40.and, at some selected high value of average current, will make with the contact 40. Similarly, upon a decrease in average current through coil 37, the armature 39. will approach thecontact 41 and, at some selected low value of average current, will make with the contact 41.

Contact 40 is connected to. ground potential, while contact 41 is connected to B+. Armature 39 is connected to one side of the coil of a relay 42 through a locking coil 43 of the meter relay 38. and a normally closed reset switch 44. The other end of the coil of relay 42 is connected to the center of a voltage divider formed by series-connected resistors 45 and 46 connected between ground and B+. The relay 42 has an armature 47, a normally closed contact 48 and a normally open contact 49 which may be connected to local alarm-giving mechanisms but which preferably will be connected to a conventional signaling transmitter coupled to a remote central station.

When. the average current flow through the coil 37 exceeds the selected high value, the relay 42 will be energized by having one end of its coil connected to ground through switch 44, coil 43, armature 39 and contact 40. When the average current flow through coil 37 falls below the selected low value, the relay 42 will be energized by having one end of its coil connected to 13+ through switch 44, coil 43, armature 39 and contact 41. With The anode of tube 34.

the relay 42 energized, contact 48 will become open and contact 49 will become closed, resulting in transmission of an alarm signal. If desired, relay 42 may be of the polar type so that in the absence of energizing current its armature will assume a median position and with current of one polarity the armature will make with cont-act 48 while with current of the other polarity the armature will make with contact 49.

When armature 39 makes with either contact 40 or 41, current fiow through locking coil 43 will hold the meter relay 38 and the alarm relay 42 in operated condition until reset switch 44 is manually opened to return the system to normal.

Each electrical impulse supplied to the grid of tube 27 from Geiger tube 22 is amplified in tube 27, is further amplified in tube 34, and causes a pulse of increased current flow through coil 38. The grid bias of tube 34 may be adjusted so that a small steady state current will flow through coil 37, or it may be adjusted so that tube 34 will be cut off in the absence of input voltage impulses.

With the normal radiation from source 19 incident on Geiger tube 22, the impulses of current flowing in coil 37 will maintain the armature 39 in its central position. This is a result of the integrating effect of the mechanical inertia of the meter relay 38 aided by the integrating effect of a capacitor 50 connected between the anode of tube 34 and 3+; or, in other words, connected across the series connection of resistor 36 and coil 37. The arma ture 39 may be adjusted to its middle position with the source 19 and opening 23 in normal alignment.

When the door is moved by some predetermined amount, the radiation incident on Geiger tube 22 from source 19 will fall to a level at which the relay 42 will be operated to transmit an alarm. The predetermined door motion necessary for an alarm may be adjusted by means of the level control 35 and by adjusting the contact 41. Depending upon circumstances, a door motion to cause alarm will usually be selected at some small value, e.g., one half inch, but more permissible motion may in some cases be desirable, especially with very large doors.

The relay 42 will also be energized and caused to give an alarm if an extra radioactive source is brought too close to the Geiger tube 22 in an efiort to jam the systern.

If desired, the relay 42 may be arranged so as to be normally energized and to be deenergized upon an alarm. In this way an alarm would be transmitted in the event of a power failure. Alternatively, an additional relay may be provided to transmit an alarm upon a power failure or upon tampering with the cabinet containing the electronic'equipment.

The circuit of FIG. 4 may, of course, be rearranged in many ways and considerably different circuits may be used. By way of illustration only, typical values for the major elements of FIG. 4 will be set forth; it should be understood that these values are not to be considered a limitation of the invention.

Table of typical values While the invention has been described in connection 6 with a specific embodiment thereof and in a specific use, various modifications thereof will occur to those skilled in the art without departing from the spirit and scope of the invention as set forth in the appended claims.

What is claimed is:

1. A burglar alarm system for signalling opening movement of a building closure, comprising a radioactive source mounted on said closure, a Geiger counter mounted on a fixed part of the building so as to be in alignment with said source when said closure is in its closed position, a radiation shield disposed about said Geiger counter and having an opening arranged to limit the angle of incidence of radiation contacting said Geiger counter, means to average the electrical impulse output of said Geiger counter over a brief interval of time to provide an average signal output level, and means responsive to a change in said average output level above or below predetermined limits to give an alarm signal indication.

2. A burglar alarm system for signalling opening movement of a roll curtain door, comprising a radioactive source mounted on said door, a radiation detector having an output comprising electrical imputlses proportional to the strength of radiation incident on said detector and being mounted on a fixed part of the building so as to be in alignment with said source when said door is in its closed position; a radiation shield disposed about said detector and having an opening arranged to limit the angle of incidence of radiation contacting said detector; means to average said electrical impulse output of said detector over a brief interval of time to provide an average signal output level, and means responsive to a change in said average signal output level above or below predetermined limits to give an alarm signal indication.

3. A burglar alarm system for signalling opening movement of a roll-curtain door or the like, comprising a radioactive source mounted on said door adjacent an edge thereof, a radiation detector mounted on a fixed part of the building adjacent said door so as to be in alignment with said source when said door is in its closed position, a shield surrounding said detector and having an opening arranged to admit radiation from said source in a narrow range of angles of incidence, a relay meter coupled to the output of said detector and arranged to maintain its armature in a median position in response to radiation from said source contacting said detector with said door in its closed position, said armature being arranged to move in one direction when the radiation incident on said detector decreases and in the other direction when the radiation incident on said detector increases, and means responsive to predetermined motions of said armature in either of said directions to transmit an alarm signal indication, said relay meter having sufficient mechanical inertia to produce a substantial averaging effect upon said detector output.

4. A burglar alarm system for signalling opening movement of a roll-curtain door or the like, comprising a radioactive source mounted on said door adjacent an edge thereof, a Geiger tube mounted on a fixed part of the building adjacent said 'door so as to be in alignment with said source when said door is in its closed position, a shield surrounding said Geiger tube and having an opening arranged to admit radiation from said source in a narrow range of angles of incidence, a relay meter coupled to the output of said Geiger tube and arranged to maintain its armature in a median position in response to radiation from said source contacting said Geiger tube with said door in its closed position, said armature being arranged to move in one direction when the radiation incident on said Geiger tube decreases, and in the other direction when the radiation incident on said Geiger tube increases, means responsive to predetermined motions of said armature in either of said directions to transmit an alarm signal indication, said relay meter having suificient mechanical inertia to produce a substantial averaging efiect upon said voltage impulses, and a capacitive element connected in parallel with said relay meter to produce an additional averaging effect upon said voltage impulses.

5. A burglar alarm system for signalling opening movement of a roll-curtain door or the like, comprising a radioactive source mounted on said door adjacent an edge thereof, a Geiger tube mounted on a fixed part of the building adjacent said door so as to be in alignment with said source when said door is in its closed position, a shield surrounding said Geiger. tube and having an open ing arranged to admit. radiation from said source a.

narrow range of angles of incidence; means to amplify armature being arranged to move in one direction when the radiation incident, on said Geiger: tube decreases, and

in theother direction when the radiationiincident on said: 20

Geiger tube increases, means responsive to predetermined motions of said armature in either of said directions to transmit an alarm signal indication, said relay meter having sufficient mechanical inertia to produce a substantial averaging efiect upon said voltage impulses, and a capacitive element connected in parallel with said relay meter to produce an additional averaging effect upon said voltage impulses.

References Cited in the file of this patent UNITED STATES PATENTS 2,383,478 Friedman et al Aug. 28, 1945 2,456,233 Wolf Dec. 14, 1948 2,474,271 Meyer June 28, 1949 2,499,889 Teichmann Mar. 7, 1950 2,646,556 Allen July 21, 1953 2,721,276, Exner Oct. 18, 1955 2,747,177 Keller May 22, 1956 2,874,305 Wilson Feb. 17, 1959 

